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Observing Galaxy Evolution in the Context of Large Astro2020 Science White Paper Observing Galaxy Evolution in the Context of Large-Scale Structure Thematic Areas: Planetary Systems Star and Planet Formation Formation and Evolution of Compact Objects Cosmology and Fundamental Physics Stars and Stellar Evolution Resolved Stellar Populations and their Environments 7Galaxy Evolution Multi-Messenger Astronomy and Astrophysics Principal Author: Name: Mark Dickinson Institution: NOAO Email: [email protected] Phone: 520-318-8531 Co-authors: Yun Wang (Caltech/IPAC), James Bartlett (NASA JPL), Peter Behroozi (Arizona), Jarle Brinchmann (Leiden Observatory; Porto), Peter Capak (Caltech/IPAC), Ranga Chary (Caltech/IPAC), Andrea Cimatti (Bologna; INAF), Alison Coil (UC San Diego), Charlie Conroy (CfA), Emanuele Daddi (CEA, Saclay), Megan Donahue (Michigan State University), Peter Eisenhardt (NASA JPL), Henry C. Ferguson (STScI), Karl Glazebrook (Swinburne), Steve Furlanetto (UCLA), Anthony Gonzalez (Florida), George Helou (Caltech/IPAC), Philip F. Hopkins (Caltech), Jeyhan Kartaltepe (RIT), Janice Lee (Caltech/IPAC), Sangeeta Malhotra (NASA GSFC), Jennifer Marshall (Texas A&M), Jeffrey A. Newman (Pittsburgh), Alvaro Orsi (CEFCA), James Rhoads (NASA GSFC), Jason Rhodes (NASA JPL), Alice Shapley (UCLA), Risa H. Wechsler (Stanford/KIPAC; SLAC) Abstract: Galaxies form and evolve in the context of their local and large-scale environments. Their baryonic content that we observe with imaging and spectroscopy is intimately connected to the properties of their dark matter halos, and to their location in the “cosmic web” of large-scale structure. Very large spectroscopic surveys of the local universe (e.g., SDSS and GAMA) measure galaxy positions (location within large-scale structure), statistical clustering (a direct constraint on dark matter halo masses), and spectral features (measuring physical conditions of the gas and stars within the galax- ies, as well as internal velocities). Deep surveys with the James Webb Space Telescope (JWST) will revolutionize spectroscopic measurements of redshifts and spectral properties for galaxies out to the epoch of reionization, but with numerical statistics and over cosmic volumes that are too small to map large-scale structure and to constrain halo properties via clustering. Here, we consider advances in understanding galaxy evolution that would be enabled by very large spectro- scopic surveys at high redshifts: very large numbers of galaxies (outstanding statistics) over large co-moving volumes (large-scale structure on all scales) over broad redshift ranges (evolution over most of cosmic history). The required observational facility can be established as part of the probe portfolio by NASA within the next decade. 1 1 Introduction The initial conditions for galaxy formation come from the quantum fluctuations that were imprinted on the dark matter distribution during inflation. The large-scale distribution of galaxies results from the gravitational evolution and hierarchical clustering of these fluctuations. In today’s era of precision cosmology, we believe that we understand the growth of structure in a universe of cold dark matter and dark energy, and can map this over cosmic time with sophisticated numerical simulations. However, the galaxies that we see are not simply dark matter halos. Baryonic physics makes them far more complex, and we are still far from understanding how galaxies form and develop in the context of an evolving “cosmic web” of dark matter, gas and stars. Gas flows along intergalactic filaments defined by the skeleton of the dark matter distribution. It cools and condenses into dark matter halos, forming stars that produce heavy elements that further alter the evolutionary history of the baryons. A full understanding of galaxy evolution will not emerge until models are constrained by rich observational data over a broad swath of cosmic history, during which galaxies and cosmic structure were growing together. Spectroscopy for very large galaxy samples over large cosmic volumes is critical for achieving this full understanding. In the local (z ∼ 0) universe, this has been amply demonstrated by very large, rich spectroscopic surveys like SDSS and GAMA. Spectra provide diagnostics of important physical properties of gas and stars within galaxies, while also precisely locating those galax- ies within their environmental context. Large surveys enable robust statistical analyses that span a wide range of galaxy properties (type, mass, morphology, star formation rate, chemical abun- dance, etc.) and environment (from voids to rich clusters). At higher redshifts (z ∼ 0:7), surveys like zCOSMOS (Lilly et al., 2009), PRIMUS (Coil et al., 2011) and VIPERS (Guzzo et al., 2014) have begun to explore galaxies in the context of large-scale structure. At still higher redshifts, deep “pencil-beam” spectroscopy on 8-10m telescopes measures redshifts out to z ≈ 8, but with sample sizes and survey volumes that are orders of magnitude too small to robustly connect galaxy evo- lution to environment. DESI, PFS and MOONS will measure large samples over large volumes, but these are (primarily) optical instruments: the number of galaxies measured will drop steeply at z 1, and these instruments will make only limited measurements of the optical rest-frame nebu- lar emission lines that are critical for measuring ISM conditions or kinematics, let alone continuum and absorption lines to diagnose stellar population properties. Slitless near-infrared spectroscopy from Euclid and WFIRST can survey large volumes, but with limited sensitivity and wavelength coverage and very low spectral resolution, leading to a very blurry view of the 3D galaxy dis- tribution over restricted ranges of redshift (Fig. 1). JWST NIRSpec will revolutionize deep field spectroscopy with full wavelength coverage from 1 to 5 µm, measuring optical rest frame emission lines out to the epoch of reionization, but only over very narrow fields and in samples of hundreds or thousands, not millions. Here, we consider the advances in our understanding of galaxy evolution that could be achieved with a “time lapse SDSS” that obtains spectra for vast numbers of galaxies over very large volumes spanning most of cosmic history. 2 Galaxy Evolution and the Cosmic Web Galaxy properties correlate with those of the underlying dark matter halos: their masses, spins, and positions within the cosmic web. The challenge of deriving dark matter halo masses for high 2 redshift galaxies is best met with very large spectroscopic surveys that can measure galaxy clus- tering for galaxies binned by many other observable/inferable parameters, such as stellar mass, star formation rate, or morphology (Fig. 2). The stellar mass—halo mass relationship (SMHMR) (e.g., Moster et al., 2013; Behroozi et al., 2013) measures the efficiency with which galaxies turn incoming gas into stars, and is a key probe of the strength of feedback from stars and supermas- sive black holes. Today we have only rough estimates of the SMHMR from abundance matching, clustering, and weak lensing (Behroozi et al. 2013, Kravtsov et al. 2013, Hudson et al. 2015). Very large spectroscopic surveys can measure redshift evolution of the SMHMR for massive halos (bias b & 1, Fig. 3a) as a function of galaxy properties, which is impossible to do via abundance match- ing. It is also clear that the SMHMR does not capture the entire influence of dark matter on galaxy properties. For example, galaxy star formation rates correlate strongly with halo growth rates, a result which requires spectroscopic environmental measures to constrain (Behroozi et al., 2018). Additional constraints can be provided by group catalogs, wherein halo masses are mea- sured for individual galaxies based on spectroscopically-identified satellite galaxy counts (Fig. 3b). These catalogs will also provide information about whether a given galaxy is a satellite or not; this is important for interpreting whether galaxy properties arise mainly from internal processes or instead from interaction with a larger neighbor. Other open questions in galaxy formation concern how halo assembly affects galaxy properties and supermassive black hole activity. Halo assembly is not directly measurable, but many assembly properties (e.g., concentration, spin, mass accretion rate) correlate strongly with environmental density (e.g., Lee et al., 2017). Large spectroscopic surveys to z ∼ 3 or beyond will be able to measure environmental densities (Fig. 3c). They can also measure average dark matter accretion rates for galaxies via the detection of the splash-back radius (a.k.a., turnaround radius) of their satellites as in More et al. (2016) out to z ∼ 5 (Fig. 3d). 3 Black Holes, AGN, and Feedback The shape of the SMHMR implies a characteristic mass at which galaxies most effectively convert gas into stars. At lower and higher masses, “feedback” is invoked to prevent gas from cooling onto galaxies, or to expel gas, thus suppressing star formation efficiency. The most massive galaxies today are predominantly quiescent, with little active star formation, and with dominant bulges that universally contain supermassive black holes. These, when fueled, can power active nuclei which may dump tremendous energy into their environments. AGN feedback, perhaps coupled with environmental effects, may be the dominant process regulating star formation and growth for massive galaxies. Like cosmic star formation, the luminous output of AGN and (by inference) the black hole growth rate, peaked at high redshift. Spectroscopy at 1-4 µm
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